The invention generally relates to a communication network. More specifically, the invention relates to reducing Peak to Average Power Ratio (PAPR) in an Orthogonal Frequency Division Multiplex (OFDM)/Orthogonal Frequency Division Multiplex Access (OFDMA) communication network.
In an OFDM/OFDMA communication network, certain combinations of OFDM symbols may result in signals with a large PAPR. One or more signals that have a large PAPR may get clipped in the RF Power Amplifier (PA). Therefore, the RF PA has to operate linearly for a wide range of amplitudes, such that, clipping of one or more signals that have a large PAPR is prevented. Moreover, the signals that have large PAPR saturate the RF PA and a transmitter, as a result of which, data in the OFDM/OFDMA communication network is lost. Additionally, transmitting the signals that have large PAPR reduces the power efficiency of the transmitter. A reduction in the power efficiency, further, reduces the battery life of mobile devices in the OFDM/OFDMA communication network. Examples of the mobile devices may include, but are not limited to, cell phones, and Personal Digital Assistants (PDA).
In conventional applications, PAPR of a plurality of OFDM symbols is reduced by digitally clipping one or more of OFDM symbols to a predetermined level before transmission. The digital clipping of the signals reduces unintentional clipping of one or more OFDM symbols in the RF PA before transmission. However, the digital clipping before transmission results in spectral spreading, which has to be filtered to decode the data carried by the plurality of OFDM symbols. Moreover, if EVM requirement of constellations of the plurality of OFDM symbols is considerably strong, then PAPR of the plurality of OFDM symbols is not reduced.
Further, in some conventional applications, a predefined set of tones in a plurality of OFDM symbols is reserved for carrying data. Thereafter, data transmitted on the predefined set of tones is decoded at a receiver, such that, PAPR of the plurality of OFDM symbols is reduced. However, decoding of data on the predefined set of tones is computationally complex.
In some conventional applications, constellation manipulation is used at a transmitter to reduce PAPR of a plurality of OFDM symbols. In constellation manipulation, for example, a phase of the plurality of OFDM symbols is modulated to reduce PAPR. The phase of the plurality of OFDM symbols is modulated using Partial Transmit Sequences (PTS) and Selective Mapping (SLM) methods. In these methods, a phase of sub-blocks of the plurality of OFDM symbols is modulated. Additionally, in these methods, a data frame of the plurality of OFDM symbols is multiplied with random vectors to reduce PAPR. However, in these methods, a receiver is required to know the type of constellation manipulation carried out at the transmitter. Further, the receiver has to violate one or more standard specifications in the OFDM/OFDMA communication network to know the type of constellation manipulation carried out in the transmitter.
Additionally, some conventional applications use codes to reduce PAPR of the plurality of OFDM symbols. Applications may use block-coding methods. In the block-coding method, a polyphase sequence that has small PAPR is transmitted for each OFDM symbol constellation. Block-coding method requires the use of large look-up tables for encoding and decoding of the plurality of OFDM symbols. Additionally, block-coding method requires an exhaustive search to identify one or more OFDM symbols that have poly-phase sequence with small PAPR. Further, conventional applications using codes detect one or more OFDM symbols that have less PAPR in an unstructured way.
Further, specific codes, for example, co-sets of Reed-Muller codes, may be used by some conventional applications to reduce PAPR of a plurality of OFDM symbols. However, the use of the specific code is limited to a small number of OFDM symbols.
There is therefore a need for a method and system that reduces PAPR of a plurality of OFDM symbols in a computationally efficient way, which is easy to implement. Further, the method and system should be independent of one or more of source coding, employed modulation, pilot sub-carrier insertion, and equalization methods used by a receiver in the OFDM/OFDMA communication network. Therefore, the method and system should be applicable to existing implementations and standard specifications. Additionally, the method and system should support multiple degrees of freedom, which can be employed to reduce the number of computations and/or processing load.